a little more on this slur...
I live in a community in the Northeast that's mostly Salvadoran, Honduran, Guatemalan, Dominican, and Puerto Rican, but with relatively few Mexicans or Cubans. And I have almost never heard that term "wetback". To me it's a term used by racist Anglos to specificy designate Mexicans, especially illegals.
It’s possible, though, that in other parts of the USA, especially the South and West, it may be used in reference to all Hispanics.